LibC: Add most of the "header may make visible" dependencies from POSIX

Author: https://github.com/timschumi
Commit: aae106e37b
Pull-request: https://github.com/SerenityOS/serenity/pull/20044
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/BertalanD

Userland/Libraries/LibC/fcntl.h

@@ -7,6 +7,11 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html
+#include <sys/stat.h>
+#include <unistd.h>
+
 #include <Kernel/API/POSIX/fcntl.h>
 #include <Kernel/API/POSIX/sys/stat.h>
 #include <sys/cdefs.h>